The paper is focused on the comparison of selected properties of the tension and normal wood. Beech is wood with high frequency of defects such as red false ...
ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 60(1): 31−40, 2018 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2018.60.1.04
THE EFFECT OF TENSION WOOD ON THE SELECTED PHYSICAL PROPERTIES AND CHEMICAL COMPOSITION OF BEECH WOOD (FAGUS SYLVATICA L.) Tatiana Vilkovská – Ivan Klement – Eva Výbohová ABSTRACT The paper is focused on the comparison of selected properties of the tension and normal wood. Beech is wood with high frequency of defects such as red false heartwood, reaction wood (tension wood), dote and so forth. The quality of beech wood is determined according to the structure and properties. The tension wood is considered an important wood defect because it causes negative alterations in solid wood quality and limits an industrial utilization of the wood. Tension wood content in our research was evaluated by the initial longitudinal warping and the woolly appearance of surface. The wood turning blanks of normal and tension wood were selected with a thickness of approximately 50 mm, width of 55 mm, and length of 460 mm. Quantity of 32 woodturning blanks were determined for moisture content and 16 samples for density from two groups tension [TW] and normal wood [NW]. Fourier transform-infrared (FTIR) spectroscopy measurements were carried out using a Nicolet iS10 FTIR spectrometer equipped with Smart iTR attenuated total reflectance (ATR) sampling accessory with diamond crystal (Thermo Fisher Scientific). A resolution of 4 cm 1 and 32 scans per sample was used. Higher variations in average moisture content were in the samples of tension. No significant statistical differences were found between density of tension and normal wood samples measured in fresh state. The cellulose in tension wood has higher degree of crystallinity than in normal wood. Key words: tension wood, normal wood, FTIR, moisture content, chemical composition of tension wood.
INTRODUCTION Tension wood has different anatomical and also chemical characteristics than normal (opposite) wood. The tension wood occurs largely in beech wood. KÚDELA and ČUNDERLÍK (2012) stated 14 up to 21 % ratio of tension wood in beech wood. Due to tension wood occurrence, the consequences are shown in form of deformations, increased portion of waste, and decreased quality of final products (KLEMENT, VILKOVSKÁ 2016). KÚDELA and ČUNDERLÍK (2012), YAMAMOTO et al. (2005) studied an influence of tension wood on material moistness. Authors stated that increased portion of cellulose in tension wood, where by weaker bonding between G-layer and S2 layer bigger swelling of other layers occurs and therefore, another sorption sites are created, as the main reason of its possible higher moisture content (MC). CHAFE (1990), ARGANBRIGHT et al. (1970), 31
COUTAND et al. (2004) studied the difference in density in fresh state of tension and normal wood. According to TARMIAN et al. (2012), PILATE et al. (2004), density of tension wood is about 5 up to 10 % higher than normal wood measured in fresh state. The density in ADS does not need to be different but based on made analysis, the density of tension wood is slightly higher (KÚDELA and ČUNDERLÍK 2012; BARAŃSKI et al. 2017, DELIJSKI et al. 2015, 2016, DZURENDA, DELIJSKY 2012). WASHUSEN et al. (2001), CLAIR et al. (2001, 2003) observed that variability in density in oven-dry method is considerable in differently developed tension wood. Different microscopic and sub-microscopic structure influences physical and mechanical properties to a large extent. CLAIR et al. (2006) described the tension wood chemical composition and claimed that the tension wood is mainly composed of high portion of crystalline cellulose. According to MATTHECK and KUBLER (1995), due to the composition of the cell wall and its micro and submicroscopic structure, the cell wall can be established as a reinforced matrix, which is mainly composed of polymers and where microfibrils angle gives needed stiffness to increased crystalline cellulose content. The difference in density is conditioned by percentage of G-layer. JOUREZ et al. (2001) observed the difference in poplar wood (Populus nigra L.). They concluded that increased density is not based only on cell wall thickness bun also on cell count and diameter in tension wood. The physical and mechanical properties of polymers are profoundly dependent on the degree of crystallinity (MO et al. 1994, GEFFERTOVÁ 2016 et al., KAČÍK et al. 2016). KAČÍKOVÁ (1997) studied crystallinity of tension and opposite wood and observed the higher portion of crystalline cellulose in tension wood. The measured infrared spectra of cellulose and calculated ratios of absorbance at wavenumbers A1108/1091 cm 1 and A1430/1043 cm1 are in Tab. 1. It can be concluded that absorbance ratios of cellulose infrared spectra are higher in tension wood than in normal wood. It follows that cellulose crystallinity in tension wood is higher than in opposite. Chemical analysis showed higher percentage of cellulose in tension wood 8–33 % and lower percentage of lignin 19–26 % and pentosanes 16–22 %. Tension wood is characterized by higher average degree of polymerization (DP) of cellulose (KAČÍKOVÁ 1997). Tension wood with highly developed G-layer contains lower percentage of pentosanes and about 10 % more glucoses than normal wood. The more developed the tension wood, the less lignified the G-layer (KÚDELA and ČUNDERLÍK 2012). Differences in the chemical composition of reaction (tension) and normal wood may be reflected in the chemical process of the raw material. Based on the cited work Kačíková (1997) reported that a lower content of lignin in the material results in a rapid delignification, reducing the time of the pulping and in a lower content of residual lignin in the obtained pulp. The main objective of the research is focused on comparison of selected physical properties and chemical composition of the tension and normal wood. Tab. 1 Estimation of cellulosic crystallinity (KAČÍKOVÁ 1997). Wavenumbers [cm-1]
TW1
NW1
TW2
NW2
A 1108/1091 A 1430/1403 A 1459/1403
1.379 1.0392 1.0081
1.0552 1.0362 1.0053
1.0562 1.0723 1.0159
1.0571 1.0708 1.0136
32
MATERIAL AND METHOD Beech wood (Fagus sylvatica L.) was used for experimental measurements. Samples were chosen from beech logs with the size of 400 mm in diameter and 2,000 mm in length from Kronotimber Ltd. (Lehota pod Vtáčnikom, Slovakia). It is most produced diameter in this company. Logs without visible defects such as red false heartwood and dote were selected. The tension wood content were evaluated by the initial longitudinal warping and the woolly appearance of surface (Fig. 1).
Fig. 1 Initial longitudinal warping and the woolly appearance surface of tested timbers.
The test timbers of normal and tension wood were selected with a thickness of approximately 50 mm, width of 55 mm, and length of 460 mm. There were 32 test timbers used in the dry kiln from two groups Tension [TW] and Normal [NW] wood. From the selected test timber, samples were cut to 20 mm in length (Fig. 1) and prepared for measured moisture content [MC]. The gravimetric method was used to determine the moisture content (MC). The moisture content of the wood samples was determined before and after drying as well as after the steaming process. The moisture content was calculated using Eq. 1, m m0 (1) MC w .100[%] m0 Where: mw is the weight of moisture sample (g) and mo is the weight of oven-dry sample (g). Density in the fresh state was determined on every second sample of the tension timber and normal (16 samples) according to EN 49 0108. The measurement was carried out under laboratory conditions. Density in fresh state was calculated using Eq. 2,
w
mw Vw
[kg∙m3]
(2)
Where: mw is the weight of moisture sample in fresh state (kg) and Vw is the volume of moisture sample in fresh state (m-3). Fourier-transform infrared spectroscopy (FTIR) is very useful analytical method for analysing the structure of wood constituents. Attenuated total reflectance (ATR) is nondestructive method enabling to observe the differences in wood chemical composition, and structure of wood constituents at different sample sites. These facts made us use the ATRFTIR to compare the chemical structure of tension and opposite wood. Fourier transform-infrared (FTIR) spectroscopy measurements were carried out using a Nicolet iS10 FTIR spectrometer equipped with Smart iTR attenuated total reflectance 33
(ATR) sampling accessory with diamond crystal (Thermo Fisher Scientific). Spectra were measured in the wavenumber range from 4,000 up to 650 cm1. A resolution of 4 cm1 and 32 scans per sample was used. Six measurements were performed per sample (Fig.2). Spectra were evaluated using the OMNIC 8.0 software (Thermo Fisher Scientific). From obtained spectra, the average spectra for each sample were created and evaluated. Normalization was performed on the absorption band at 1,033 cm1. Measured FTIR spectra were used to estimation, of chemical composition and cellulose crystallinity in tension and normal wood. Following ratios were used for cellulose crystallinity estimation: o A1370/A2900 (TCI) – total crystallinity index (O´CONNOR et al. 1958) o A1424/A898(LOI) – lateral order index (O´CONNOR et al. 1958) o A1335/A1315 (COLOM et al. 2003)
Fig. 2 a.) Samples for FTIR analyses, b.) Apparatus Nicolet iS 10 (Thermo Fisher Scientific).
RESULTS AND DISCUSION Mean values of moisture content and elemental statistical characteristics for tension and normal samples are shown in Tab. 2. Moisture content range from 41.7–69.5 % for normal wood samples and from 43.6–81.6 % in tension wood samples, respectively. The ranges had higher values in tension wood (TW). Equally, the average final moisture content was about 0.52 % higher in tension wood, what can be considered a negligible difference in practice (production conditions). Analysis of MC by basic statistical characteristics twofactor t-test confirmed that there were no considerable statistical changes between measured differences between tension and normal wood. Tab. 2 t-Test Two-Sample for Variances of Moisture content (NW and TW). NW 58.161 48.223 32.000 0.000 56.000 0.250 0.402 1.673 0.803 2.003
Mean Variance Observations Hypothesized Mean Difference df t Stat P(T
